Among the most highly expressed drug transporters in neonatal organs are the organic cation transporters (Octs), which transport drugs, neurotransmitters, metabolites and toxins. Many drug-drug interactions and metabolic derangements due to drugs are thought to occur at the level of the transporter, and certain polymorphisms can lead to drug toxicity. Oct1/SLC22A1 is the prototypical member of the SLC22A family of transporters and along with certain ABC transporters (e.g., MDR), Oats and Oatps, is one of the most important xenobiotic (phase III drug transporter) transporters in the body. The PI's lab identified Organic Anion Transporter 1 (Oat1/NKT/Slc22a6), which, along with Oct1 originally helped define the SLC22 transporter family. We have studied the in vivo function of several SLC22 transporters and performed detailed in vitro analysis and in silico modeling of substrate-transporter interactions. Nevertheless, detailed molecular analyses of substrate-transporter interactions-which are necessary to take a rational approach to diminishing adverse affects of drugs transported by Oct1-are limited by the absence of precise structural information. Here we propose to combine our knowledge of SLC22 transporter biology (and skill with in vitro, ex vivo and in vivo assays) together with the expertise of top-notch investigators in x-ray crystallography, protein and substrate modeling to investigate the molecular mechanisms of the key substrate-transporter interactions involved in Oct1-mediated organic cation xenobiotic handling. Although in this proposal we focus on Oct1, the results will also provide important information relevant to other SLC22 drug transporters. To our knowledge, we are among the very few groups in this field that has a proven ability to combine such diverse areas of expertise, including computationally biology, structural biology and in vivo/in vitro wet-lab biology and is one of the most innovative aspects of the proposed studies. We will crystallize and determine the high-resolution x-ray structure of Oct1 (SA1) in a continuing collaboration with Dr. Geoffrey Chang (involving both his laboratory and TransportPDB, a component of the NIH Protein Structural Initiative of which he is a founding member) to gain key insights into the structural basis of substrate binding and transport. We will use our documented expertise in ligand-based and transporter-based computational approaches (SA2) to identify the molecular determinants of substrates and the Oct1 protein mediating their interactions. Pharmacophore-based virtual screening of chemical structure libraries will be done, and identified structures will be used for molecular dynamic analyses to prioritize compounds. Identified compounds will be obtained and tested in wet-lab studies using in vitro, ex vivo and in vivo approaches well established in the PI's group (SA3). This multifaceted structural- computational-wet-lab strategy will result in major advances in understanding adverse drug reactions in children as well as adults and help move the drug transporter field forward.